Push-pull connection for amplifying electric oscillations



July 31, 1934. K. POSTHUMUS 1,968,260

PUSH-PULL CONNECTION FOR AMPLIFYING ELECTRIC OSCILLATIONS Filed May 12, 1932 INVENTOR KLAAS P05 HUMUS ATTORNEY Patented July 31, 1934 PATENTAOFFICE PUSH-PULL ooNNEo'rioN FOR AMPLIFYING ELECTRIC OSCILLATIONS Klaas Posthumus, Eindhoven, Netherlands, as-

signor to Radio Corporation of America, a corporation of Delaware 1 Application May '12, 1932, Serial No. 610,767

- In the Netherlands May 5, 1931 4 Claims. (Cl. 179-171) For amplifying electric oscillations of very high frequencies often the so-called push-pull connection is used, in which for compensating the capacity between plate and grid of both valves neutralizing condensers are inserted in lines by which said plates andv grids are crosswise con-.-

nected.

It is endeavored tcconstruct the circuit arrangement in such a manner that the neutralizing is independent of the frequency, since otherwise there is a risk that apart from the wave to be amplified one or more spontaneously generated waves will occur.

This result could already be obtained with the means known up till now, if valves could be used between the filaments of which no impedance appears. If, however, large quantitiesof energy are dealt with one is practically compelled to use large valves which are spaced apart from each other. and in which the cathodes are interconnected by a line. This is especially .the case when using two pairs of valves and if it is required to be able to change over to other wave lengths.

In such cases it has been found that it is often extremely difilcult to achieve a sufiicient stability and furthermore that the cause of the instability is due to the impedances (mainly the inductance) of the lines interconnecting the electrodes of both valves of the.push-pullconnection, .viz. on one hand both cross lines grid-neutralizing condenserplate, and on the other handcthe line between both filaments. 1

According to the invention a perfect stability independent of; thefrequency may be obtained, by giving the impedances (self-inductions) referred to above a ratio substantially corresponding to the ratio existing between the two capacities plate-filament and plate-grid of the valves respectively.

The invention will be better understood by reference to the accompanying drawing, representing, by way of example, one embodiment thereof.

Fig. 1 shows the well-known diagram of connections of a neutralized push-pull amplifier.

Fig. 2 represents the diagram according to Fig. 1 but shown as a Wheatstone bridge.

Fig. 3 shows the same diagram but modified in accordance with the invention.

Fig. 4 shows a spare diagram of Fig. 3 from which clearly appears the principle of the invention.

In Fig. 1 both push-pull connected valves are designated by V1 and V2. Between their grids G1 and (312 connected a tunable input circuit I and between their anodes A1 and A2 is mounted a.

tunable output circuit II. The anodes A1 and A2 are connected to the grids G2 and G1 respectively through neutralizing condensers Cn. One pole of both filaments F1 and F2 are interconnected through a short conductor. 7

For the sake of clearness the sources of heating current and of anode current have not been represented.

The diagram of Fig. 1 may be replaced by the bridge connection shown in Fig. 2, in which the capacities Cag are arranged between A1 and G and between A2 and G2 respectively, whereas the capacities Cat are inserted between A1 and F1 and between A2 and F2 respectively. This figure shows furthermore both neutralizing condensers C11 and the impedances which in the case under View are imagined as pure inductances Z11 of both cross lines and the impedance, in this case the inductance Z1, of the line between both filaments. The input circuit I lies between the points G1 and G2 and the output circuit between the points A1 and A2.

A perfect stability which is independent of the tuning of the circuits I and II will be obtained if an oscillation occurring between both anode points A1 and A2 does not produce voltage variations between the grids and the filaments of the tubes. I

When examining whether this condition can be satisfied in the diagram shown in Figs. 1 and 2 then it will appear, supposed that the impedances in'the parallel branches A1, G2, A2 and A1, F1,-F2, A2 are respectively Za Z111, Zn and Zn, Zn, Zn, that the potential difference between: G1 and F1, as well as that between G and F will be zero foreach frequency, if the condition be satisfied: Q

Zag.ZZn.'Zn=Zaf.'Zlf.'Zaf

From this equation follows when cag=cn that ZagfZln ZafiZlf O1 caglcaflfizn If the impedance cannot be supposed to be purely inductive then the above equation holds without prejudice good for the impedances Zr and Zn respectively of the lines. Consequently the above ,9 mentioned condition may be satisfied in a simple manner.

An improvement of the arrangement according to the invention can be obtained by taking care that the oscillations between the points A1 and A2 7 do not produce current variations in the input circuit. In order to obtain thisthe points of junction of the input circuit lie preferably in the middle of the cross lines with the inductances Z11.

Since the circuit A1 F1 F2 A2 is alive and both cathodes F1 and F2 have the inductance Zr between them an alternating voltage will be set up between said cathodes so that in accordance with the first-mentioned condition the same alternating voltage will be setup between both grids G1 and G2. Thus in the grid circuit I is produced an oscillation having the same frequency as that between A1 and A2 so that there is still a risk of instability.

In the diagram of connections shown in Fig. 3, of which the spare diagram is represented in Fig. 4, this risk is avoided since the connection of the input circuit I is no longer effected in the points G1 and G2 but in the middles of the selfinductions Z11. From Fig. 4 it directly appears that, since Cag=C11, that these both middles, independently of the frequency between A1 and A2, will never show a potential difference.

When determining the middles of the cross lines those parts of these lines must sometimes be taken into account, which lie between the anodes A1 and A2 and the corresponding neutralizing condensers, in case the latter are not directly united with said anodes in the way as represented schematically in Fig. 4.

I'claim:

1. An electrical network comprising a pair of electronic tubes each thereof having anode, cathode and grid electrodes, push-pull input and output circuits therefor, connections having appreciable inductances from a grid of each tube to the anode of the other tube, each of said connections including neutralizing condensers, and means for connecting the input circuit between the inductive rnidpoints of said connections whereby voltages existing across the output circuit are prevented from producing a potential difference across the input circuit.

2. An electrical network comprising a pair of electronic tubes each thereof beingprovided with anode, cathode and grid electrodes, push-pull input and output circuits therefor, connections having appreciable inductance from a grid of each tube to the anode of the other tube, each of said connections including neutralizing condensers, means for connecting the input circuit from a point of one of said connections to a point of the other thereof, said connecting points being arranged so that the inductance of the portion of the connection between the grid and the input circuit connecting point of one connection is the same as that between the neutralizing condenser and the input circuit connecting point of the other connection. 1

3. An electrical network comprising a pair of substantially alike electronic tubes each thereof being provided with anode, cathode and grid electrodes, push-pull input and output circuits therefor, connections having appreciable induct.- ance from a grid of each tube to the anode of the other tube each of said connections including neutralizing condensers, the inductance of both said connections being substantially equal, a connection also having appreciable inductance between the cathode of one tube and the cathode of the other tube, the inductance of the connection between the two cathodes bearing substantially the same ratio to the inductance of either of said first named connections as the capacity betweenthe anode and grid bears to the capacity between the anode and cathode of either tube whereby voltages existing across the output circuit are prevented from producing potential differences between the grid and filament of either of said tubes.

4. A neutralized amplifier circuit comprising a pair of electronic tubes each thereof being provided with anode, cathode and grid electrodes said tubes having substantially like characteristics, push-pull input and output circuits therefor, connections having appreciable inductance from a grid of each tube to the anode of the other tube each of said connections including a neutralizing condenser, the inductances of all of said connections being substatially equal, a connection also having appreciable inductance between the cathode of one tube and the cathode of the other tube, the inductance of the connection between the two cathodes bearing substantially the same ratio to the inductance of either of the first named connections as the ratio of the capacity between the anode and grid of either tube bears to the capacity between the anode and cathode of one of the tubes whereby voltages existing acrossthe output circuit are prevented from producing potential differences between the grid and filament of either of said tubes, and means for connecting the input circuit from a point of one of said first named connections to a point of the other of said first named connections said connecting points being arranged so that the inductance of the portion of the connectionbetween the grid and the input circuit connecting point of one connection is the same as that between the neutralizing condenser and the input circuit connecting point of the other connection.

KLAAS POSTHUMU S. 

